A micro-mirror unit is provided with a mirror face for reflecting light and can change the light reflection direction by oscillating the mirror face. An electro-statically driven micro-mirror unit which utilizes static electricity for oscillating the mirror face is adopted in many optical devices. Electrostatically-driven micro-mirror units can be classified largely as two types: micro-mirror units manufactured by so-called surface micro-machining technology and micro-mirror units manufactured by so-called bulk micro-machining technology.
In the surface micro-machining technology, on a substrate, thin films of raw materials corresponding to the individual components are processed into desired patterns, and such patterns are accumulated in order, forming individual components such as a support, a mirror face, and an electrode section which constitute the chip and a sacrificial layer which is later removed. An electrostatically-driven micro-mirror unit manufactured by such surface micro-machining technology is disclosed in JP-A-H0-7-287177 for example.
On the other hand, in the bulk micro-machining technology, a support, a mirror section, etc. are formed into desired shapes by etching the raw material substrate itself, and a mirror face and electrodes are formed as thin films as needed. An electrostatically-driven micro-mirror unit manufactured by such bulk micro-machining technology is disclosed in JP-A-H9-146032, H9-146034, H10-62709 and 2001-113443 for example.
Listed as a technical item required for a micro-mirror unit is that the flatness of the mirror face responsible for reflecting light is high. According to the surface micro-machining technology, because the mirror face which is finally formed is thin, the mirror face becomes easily curved, and high flatness is guaranteed only for those having a mirror face size of several tens of micrometers on one edge.
Conversely, according to the bulk micro-machining technology, because the mirror section is constructed by shaving the relatively-thick raw material substrate itself, and because a mirror face is installed on the mirror section, even if the mirror face has a larger area, its rigidity can be retained. As a result, a mirror face having high enough optical flatness can be formed. Therefore, in manufacturing a micro-mirror unit for which a mirror face of several 100 micrometers or longer in one edge is required, the bulk micro-machining technology is widely adopted.
Shown in FIG. 20 is a prior-art electrostatically-driven micro-mirror unit 400 manufactured by the bulk micro-machining technology. The micro-mirror unit 400 has construction where a mirror substrate 410 and a base substrate 420 are accumulated. The mirror substrate 410 comprises, as shown in FIG. 21, a mirror section 411, a frame 413, and a pair of torsion bars 412 which connect them. Installed on the surface of the mirror section 411 is a mirror face 411a. Installed on the back of the mirror section 411 are a pair of electrodes 414a and 414b. 
On the other hand, installed on the base substrate 420 are, as shown in FIG. 20, an electrode 421a opposing the electrode 414a of the mirror section 411 and an electrode 421b opposing the electrode 414b. 
With this kind of construction, if the electrode 421a of the base substrate 420 is made to be a negative pole in a state in which the electrodes 414a and 414b of the mirror section 411 are positively charged, for example, electrostatic attraction is generated between the electrode 414a and the electrode 421a, and the mirror section 411 turns in the direction of the arrow M3 while twisting the pair of torsion bars 412. The mirror section 411 turns to the angle where the electrostatic attraction between the electrodes and the total sum of the twisting resistance of the torsion bars 412 balance.
Conversely, if the electrode 421b is made to be a negative pole in a state in which the electrodes 414a and 414b of the mirror section are positively charged, electrostatic attraction is generated between the electrode 414b and the electrode 421b, and the mirror section 411 turns in the direction opposite to the arrow M3. By such oscillating driving of the mirror 411, the direction of light reflected by the mirror face 411a is switched.
As stated above, in the electrostatically-driven micro-mirror unit 400, the mirror section 411 turns to the angle where the electrostatic attraction between the electrodes and the total sum of the twisting resistance of the torsion bars 412 balance. In doing so, the degree of stress by twisting of each torsion bar 412 is not uniform in the lengthwise direction. Namely, the ends of each torsion bar 412 are connected to a movable mirror section 411 and a fixed frame 413, and when the mirror section 411 turns, the stress of twisting the torsion bar 412 concentrates to the connectors of both ends of the torsion bar 412.
As seen in FIG. 21, the width and thickness of each torsion bar 412 are constructed to be uniform. In addition, the width and thickness are set small in order to reduce the twisting resistance of each torsion bar 412 and thus reduce the driving electric power. As a result, when stress concentrates at both ends of each torsion bar 412, there is a high possibility that the torsion bar 412 will be destroyed at that spot. When the twisting angle of the torsion bar 412 (oscillating angle of the mirror section 411) is large and the twisting spring constant of the mirror section 411 is large (namely, the resonance frequency of the micro-mirror unit is high), that trend is high. Also, if the rigidity of the torsion bar 412 is uniform in the lengthwise direction, it is impossible to the meet various kinds of property requirements required of the micro-mirror unit 400.